Dehydrocyclization process



United States Patent 3,501,542 DEHYDROCYCLIZATION PROCESS Norman L. Carr, Allison Park, and Robert E. Kline and Allen E. Somers, Pittsburgh, Pa, assignors to Gulf Research 8: Development Company, Pittsburgh, Pa., a corporation of Delaware No Drawing. Filed Feb. 29, 1968, Ser. No. 709,175 Int. Cl. C07c 13/00 U.S. Cl. 260-6735 Claims ABSTRACT OF THE DISCLOSURE Nonaromatic hydrocarbons containing at least six carbon atoms in the molecule are aromatized in a dehydrocyclization process wherein said hydrocarbons are mixed with an aromatic hydrocarbon, preferably the aromatic end product of said process and this mixture is contacted with a dehydrocyclization catalyst in the presence of added hydrogen at an elevated temperature. Hexane can be converted to benzene, heptane can be converted to toluene.

BACKGROUND OF THE INVENTION This invention relates to the conversion of certain low boiling aliphatic and cycloaliphatic hydrocarbons to the corresponding aromatic compound.

The demand for aromatic hydrocarbons has been growing at a rate which threatens to outstrip available production. Aromatics are consumed in vast quantities by their use as solvents, fuels and fuel additives, and their use in the chemical industry. The Oil and Gas Journal of Sept. 5, 1966, at page 112, discusses more completely this demand for aromatic materials. As a result of the increased demand, new technology is necessary to provide an increasing volume of aromatic hydrocarbons economically.

There are several sources of aromatic hydrocarbons, one of which is a combination cyclization and dehydrogenation process, termed herein a dehydrocyclization process, wherein aliphatic hydrocarbons can be trans formed into aromatic hydrocarbons such as benzene and homologs thereof having the same number of carbon atoms as the starting material.

Certain nonaromatic hydrocarbons can be aromatized at elevated temperatures in the presence of suitable dehydrocyclization catalysts. The problems encountered with known dehydrocyclization processes include the inability to aromatize such compounds as methylpentane, methylcyclopentane, etc., excessive coke formation, poor catalyst aging, poor aromatic selectivities, and low yields. Furthermore, when n-hexane is aromatized to benzene, certain impurities in the n-hexane, such as methylcyclopentane cause excessive coking and catalyst aging and must be removed.

SUMMARY OF THE INVENTION hydrocarbon along with the preferred feedstock with a minimum of undesired side reactions when the aromatization is carried out in the presence of a significant quantity of an aromatic hydrocarbon, preferably the product aromatic hydrocarbon and hydrogen and therefore these substances may be left in the dehydrocyclization feed.

Patented Mar. 17, 1970 As starting materials, there may be used saturated or olefinic aliphatic hydrocarbons containing at least six carbon atoms in the molecule and up to about twelve carbon atoms. Exemplary of the hydrocarbons useful in the practice of the instant process are the normal paraffins such as hexane, heptane, octane, decane, and dodecane; the branched paraffins having monosubstituted methyl or ethyl groups and having at least six carbon atoms in a straight chain such as the methyl hexanes, methyl heptanes, ethyl hexanes, etc.; the branched paraffins having polysubstituted methyl and ethyl groups on different carbon atoms and having at least six carbon atoms in a straight chain, for example, the dimethyl hexanes such as 2,5-dimethylhexane; the methyl pentanes such as Z-methylphentane; the substituted cyclopentanes such as methylcyclopentane; and the monoolefinic analogs of the above such as hexene-l, 2-methylhexene-l, 2,4-dimethylhexene-l, 4 methylpentene1, 3 methylcyclopen tene-l, etc. Less preferred as feed materials are those compounds having at least six carbon atoms with less than six carbon atoms in a straight chain and having two or more substituted groups on different carbon atoms such as 2,3-dimethylbutane, 2,4-dimethylpentane, etc.; the diand triolefins such as 1,5-hexadiene, 2,5-dimethyl- 1,5-hexadiene, diisobutylene, etc. The above are less preferred because they result in poorer yields of aromatic compounds. Least desirable as feed materials are those compounds having two substituted groups on the same carbon atom such as 2,2-dimethylbutane, 2,2-dimethylhexane, etc., because they are most resistant to dehydro cyclization. The cycloaliphatic compounds having six carbon atoms in the ring such as cyclohexane, cyclohexene, methylcyclohexane, etc., when present in the aliphatic feed will readily dehydrogenate to the corresponding aromatic compound at the conditions of this process. On the other hand it is believed that such compounds as methylcyclopentane are converted to the straight chain intermediate in the reaction and therefore are also in-- volved in a dehydrocyclization reaction.

The hydrocarbon selected as the starting material will be determined in large part by the nature of the desired aromatic end product. Usually a mixture of aromatic hydrocarbons is obtained with the species and relative proportions depending in part on the feed material and the conditions of operation. Thus, a mixture of the xylenes and ethyl benzene is normally obtained from eight carbon aliphatic hydrocarbons and the desired compound or compounds can be separated from the product mixture. Where benzene is to be the end product one can select n-hexane or mixture of n-hexane with the hexenes, methylcyclopentane, 2-methylpentane, methyl pentene, etc., as the starting material. Analogous choices apply where substituted benzenes such as toluene, the xylenes and ethyl benzene are the desired aromatic end products. Thus, where toluene is desired, one can select any of n-heptane, the n-heptenes, the methyl hexanes, the methyl hexenes, and dimethylcyclopentane or a combination thereof. Where xylenes are desired, such compounds as n-octane, the n-octenes, the methylheptanes and methylheptenes, and dimethylhexanes in which the methyl groups are on different carbon atoms and either singly or in combination, can be employed. When ethyl benzene is desired, the ethyl hexanes and ethyl hexenes such as S-ethyl hexane and 4-ethyl hexene-l can be used. The instant process can also be used to upgrade by aromatization low boiling relatively aromatic-free fractions, commonly termed light gasolines, such as the C C rafliuates obtained from solvent extracted hydroformates.

We are not certain why the combination of the aromatic compound and hydrogen in the feed produces the unexpectedly superior results but we believe that the aromatic compound and hydrogen by cooperative action favorably influence both the kinetics and equilibrium behavior of the system. When either hydrogen or the aromatic compound is omitted from the feed, excessive coking, reduction of catalyst life and reduced conversions, selectivities and yields are the results. A mononuclear aromatic hydrocarbon such as benzene, toluene, the xylenes, cumene, ethyl benzene, etc., or mixtures of these can be used in the feed; however, it is preferred to use the same aromatic compound that is produced in the aromatization reaction preferably by recycling a portion of the product. We have found in the preparation of benzene, for example, that the results obtained are surprisingly superior when benzene is used in the feed rather than when a benzene homolog is used and that the latter is surprisingly superior to the absence of any aromatic compound in the feed.

Since dehydrocyclization reactions are highly endothermic, the addition of a suitable diluent should result in somewhat smaller adiabatic temperature drops, and hence a higher average reactor temperature may result in somewhat higher yields. However, we have found, surprisingly, that when both a mononuclear aromatic compound and hydrogen are added to the feed, both the conversion, that is, the total amount of aliphatic component converted and the selectivity, that is, reaction specificity for aromatic product are increased far beyond the values one would expect from thermal effects alone; increased temperatures usually decrease selectivity. Even more unique and unexpected is the fact that when the product aromatic is recycled both the reaction selectivity and the conversion increase; in the usual case an increase in selectivity is accomplished with a decrease in conversion and vice versa. In many catalytic processes, side reactions can be reduced in favor of the desired product by simply decreasing the conversion; in our process an increase both in selectivity and conversion can be obtained together with a decrease in side reactions. We have discovered that when any benzene homolog is added to the feed improved yields and selectivities result; however, the best yields and selectivities are obtained when the product aromatic is used such as by recycle.

Prior to contact with the dehydrocyclization catalyst, it is desirable to mix the nonaromatic feedstock with the desired aromatic compound, preferably the aromatic end' product of the process, in an amount such that the hydrocarbon component of the mixture contains at least about weight percent of said aromatic compound. In order to obtain significant benefits from the invention it is preferred to use at least about weight percent of the aromatic compound and more preferably at least about 40 weight percent. The upper limit may be as high as about 90 Weight percent aromatic; however, the preferred upper limit is about 70 weight percent. The broad upper limit of about 90 weight percent of aromatic in the hydrocarbon component of the feed and the preferred upper limit of about 70 weight percent of aromatic should be read in the context that, as a general rule, the overall process is enhanced by the largest amount of product recycle consistent with the limiting factor of the economics of handling and recycling large volumes of material.

At some point prior to contacting the feed with the catalyst, hydrogen is added to the system in an amount such that the mole ratio of hydrogen to total hydrocarbon feed is in the range of from about 0.1 to about 5.The preferred ratio is in the range of from about 0.5 to about 2 moles of hydrogen per mole of hydrocarbon. We have found that the addition of external hydrogen, together with an aromatic compound is a vital aspect of the instant process; otherwise, coke formation, catalyst aging and cracking rates are so great that the aromatic yields are too small to be of commercial interest. A convenient source of hydrogen is the hydrogen-rich gas formed as a by-product of the dehydrocyclization reaction and in a preferred embodiment this hydrogen-rich gas,

preferably containing at least percent hydrogen, is recycled, together with make-up hydrogen as required, and used as the source of external hydrogen. Of course, hydrogen from any convenient source can be employed with equal facility.

It must be noted at this point that the beneficial and unusual aspects of the instant process cannot be ascribed to a single independent aspect thereof. Use of the product aromatic and addition of hydrogen selectivates the reaction, improving both conversion and yield. This also contributes to a reduction in catalyst aging, reduction in coking and permits simpler reactor design in that fixed-bed adiabatic reactors can be used. When either the product aromatic or the added hydrogen is absent, the process is severely inhibited in that the results in terms of conversion and specificity which are obtained are not nearly as high as are those when both hydrogen and product aromatic are present. It is thus the combination of the aromatic end product of the process and hydrogen which accounts for the beneficial aspects of this invention.

The process defined herein can be carried out at a temperature in the range of from about 900 to about 1200 F., preferably in the range of from about 950 to about 1050 F. Since the selectivity of the process is improved by lower temperatures, it is generally desirable to adjust the other parameters of the process to enable the use of lower temperatures where possible.

We have found that the pressure is desirably kept low to maximize the aromatic yield. The pressure can range from about 2 to about 50 p.s.i.g., preferably from about 5 to 15 p.s.i.g. At pressures above 50 p.s.i.g., the yields and selectivities tend to decrease while pressures below about 2 p.s.i.g. cause operational difficulties in conducting the process and necessitate the use of larger equipment.

By the process of this invention, the manufacture of aromatic hydrocarbons has been considerably improved independent of the weight of charge per weight of catalyst and time applied. It is, however, desirable to operate at a weight hourly space velocity, based on total feed, in the range of from about 0.3 to about 4 wt./hr./wt., preferably in the range of from about 0.5 to about 1.5 wt./hr./wt.

The process as set forth herein can be operated in a variety of reactors such as, for example, fixed bed reactors operated adiabatically, or fluidized bed reactors operated isothermally. We prefer to use fixed bed adiabatic reactors. The use of product aromatic recycle permits the use of fixed-bed adiabatic reactors with inter-reactor reheat. One convenient method is the use of a series of two or more such reactors with the catalyst in certain reactors being regenerated while the others are on-stream. Each reactor can be on-stream for about 24 hours or longer depending on the specific conditions of reaction, and on regeneration for about 8 hours. The catalyst can be regenerated by burning off any combustible deposits according to conventional practice. It is the combination of aromatic recycle and added hydrogen, having the effect of reducing both catalyst aging and coking, which enables the reactors to be on-stream for such relatively long periods. The onstream periodfor each reactor will be determined primarily by the rate of aging of the catalyst.

The process of this invention can be carried out in the presence of a known dehydrocyclization catalyst. The catalysts of this type include the metal oxides of Group VI of the Periodic Table such as chromium, molybdenum and tungsten and mixtures of these oxides with one another or with one or more oxides of other metals, if desired, such as the oxides of the alkali metals, alkaline earth metals or the rare earth metals as selectivating agents. The catalyst is supported on alumina, silica-alumina, or other suitable high surface area refractory oxide support. The preferred catalyst is chromia on alumina together with a minor amount of sodium or potassium. A typical catalyst and one of the type which has been found to be effective. in the practice of this invention is one containing from about 6 to about 25 percent and preferably about 10 to about 20 percent of chromia (Cr O promoted with from about 0.1 to about 10 percent and preferably about 0.6 to about 1.5 percent sodium or potassium (as the oxide) distended on an inert carrier such as alumma.

Once the feed has passed through the reactor, the mo matic product can readily be recovered by distillation or other suitable separation procedure. In a preferred embodiment, the gases are separated first from the reactor effluent, at least a portion of which are recycled and added to the reactor feed together with make-up hydrogen as required. All of the unreacted nonaromatic feed hydrocarbons and a portion of the aromatic fraction in the liquid reactor eflluent are separated from the liquid reactor efliuent and recycled to the reactor input. The remaining portion of this aromatic fraction is recovered as the product. Where benzene is being produced, nitration grade benzene is readily obtained as the final product.

The more detailed operation of our invention is illustrated by the following examples. There are, of course, many forms of the invention obvious to one skilled in the chemical art once the invention has been revealed and it Will accordingly be understood that these embodiments are illustrative of the invention and not limitations there- EXAMPLE 1 This example illustrates the effect of benzene on the dehydrocyclization of n-hexane in the presence of hydrogen. The catalyst was 14 percent chromia distended on alumina and promoted with 0.9 percent sodium.

Note that the aromatics yield is defined as the weight of aromatic pro dued (benzene) per weight of n-hexane in the feed. The selectivity is defined as the weight of aromatics produced (benzene) per weight of nhexane consumed.

The eifect of benzene in the feed is clearly seen in the results of this embodiment. Both the net aromatics yield and the selectivity are significantly increased when the product aromatic is present in the feed.

EXAMPLE 2 This example illustrates the effect of hydrogen on the dehydrocyclization of n-hexane in the presence of benzene. The catalyst was 14 percent chromia distended on alumina promoted with 0.9 percent sodium. The pressure was increased in the run with hydrogen to provide a substantially equal partial pressure of n-hexane in each run. The total conversion and selectivity to benzene and yield of behzene for each run are set forth after one hour of operation and six and one-half hours of operation.

Without With Hydrogen Hydrogen Feed Composition, weight percent:

n-Hexane 30. 4 30. 4 Benzene 69. 2 69. 2 Conditions:

Temperature, F 1,025 1, 025

. Total pressure, p.s.i.g 3 10 Hydrogen/hydrocarbon mole ratio 0.4 Overall space velocity wt./hr./wt 1. 14 1. 14

1 hr. 6.5 hr 1 hr. 6.5 hr

Results:

. Conversion, mole percent 100 71 84. 73. 5 Decrease, percent 39 13 Selectivity to benzene, percent 83 70 80 75 Decrease, percent 16 6. 3 Yield of benzene, percent 83 42. 5 68 55 Decrease, percent 49 19 From these data it is observed that the percent decrease in conversion, selectivity and yield is much greater when the reaction is carried out without hydrogen than when it is carried out with hydrogen. The catalyst in the runs carried out without hydrogen exhibited very rapid aging and required much more frequent regeneration.

EXAMPLE 3 This example illustrates the conversion of methylcyclopentane (MCP) to benzene using the same catalyst as used in Example 1.

Conditions:

Temperature1,025 F. Total pressure8 p.s.i.g. LWHSV-0.7O Initial Hz/H-C mole ratio-0.52 Time2 hours Product C6 'olefmsnuo Conversion of MCP63.1 Weight percent. Selectivity to benzene76.7 mole percent. Net aromatic yield-46.7 mole percent.

EXAMPLE 4 Feed Product Composition, weight percent:

Hz 1. 3 2. 8 Hexenes 6. 7 7. 1 n-Hexane 26. 9 24. 6 OP- 31.7 19. 3 Benzene. 33. 4 43. 5 01-05 2. 7

Conversion of n-hexane8.5 weight percent Conversion of MCP-38.7 weight percent. Selectivity to benzene-75.8 mole percent. Net aromatics yield-18.9 mole percent.

It is thus seen, from Examples 3 and 4, that methylcyclopentane can be eifectively converted to benzene at the conditions set forth in the instant process.

This is significant in that MCP was previously an undersirable component in feedstocks and it was necessary to maintain the concentration of MCP at low levels, e.g., about 2 percent to avoid coking and premature catalyst aging. It is believed that in our process the methyl cyclopentane is converted to n-hexane and the n-hexane is then dehydrocyclized to benzene. This explains the apparent low net conversion of n-hexane in Example 4 at the relatively mild conditions utilized. The presence of analogous impurities in toluene and xylene precursors similarly has no deleterious effect.

EXAMPLE 5 This example illustrates the conversion of Z-methylpentane to benzene and the eifect of benzene on the results using the same catalyst as in Example 1.

The yield and selectivity in the run Where the product aromatic is included in the feed are significantly higher 7 than comparable values where no aromatic is present in the feed.

EXAMPLE 6 This example illustrates that benzene is a superior additive compared with o-xylene in the dehydrocyclization of an n-hexane-hexene mixture.

Feed composition, weight percent:

n-Hexane 2 Hexene-l Conditions:

Hz/H-C mole ratio Temperature, T Pressure, p.s.i.g LWHSV, wt./hr./wt.:

Total Hexane-hexene First Second First Second Results: 2 hour on-stream periods:

Selectivity: Benzene produced per hexane-hexcue consumed, weight percent 83. 6 Yield: Benzene produced per hexane-hexene in feed, weight percent It is seen that the aging rate in the run with added o-xylene is much greater than in the run with added benzene. This increased aging would account for the decreased yields and selectivities noted in theo-xylene run.

EXAMPLE 7 This example illustrates the dehydrocyclization of nheptane to toluene.

Feed Product, Liq.

Composition, weight percent:

n-Heptane Toluene.

Satnrates Toluene Yield-48.4. weight percent. Conversion-87.1 percent.

EXAMPLE 8 This example illustrates the conversion of n-heptane, contained in a 70:30 n-heptane-toluene mixture, to toluene.

Conditions:

Temperature-1,050 F. Pressure-1O p.s.l.g. LVHSV0.5 vol./hr./vol. Hz/HC1.G mole ratio Time5 hours Catalyst-14% chromia on alumina promoted with 1.4% sodium Feed Product, Liq.

Composition, weight percent:

n-Heptane 70. 3 Toluene--. 83. 8 Olefins 3. 0 Saturates 13. 2

Net yield-44 weight percent.

Conversion-83.2 percent.

Examples 7 and 8 above demonstrate that n-heptane can be converted to toluene in good yield and with a high conversion by the process of the present invention. 5

Obviously many modifications and variations of the invention, as hereinabove set forth, can be made without departing from the spirit and scope thereof and, therefore, only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. In a process for the production of aromatic compounds by the dehydrocyclization of a nonaromatic saturated or olefinic aliphatic hydrocarbon feed containing at least six carbon atoms and up to about twelve carbon atoms in the molecule wherein said nonaromatic hydrocarbon feed is contacted with a dehydrocyclization catalyst at dehydrocyclization conditions of temperature and pressure, the improvement therein which comprises admixing said nonaromatic hydrocarbon feed with:

(a) hydrogen and (b) a mono-nuclear aromatic hydrocarbon having from six to twelve carbon atoms in the molecule, and effecting said contact on said mixture.

2. A process according to claim 1 in which hydrogen is added in an amount such that the mole ratio of hydrogen to total hydrocarbon is in the range of from about 0.1 to about 5 and the mono-nuclear aromatic hydrocarbon is present in an amount of from about 10 to about percent by weight of total hydrocarbon.

3. A process according to claim 1 in which hydrogen is added in an amount such that the mole ratio of hydrogen to total hydrocarbon is in the range of from about 0.5 to about 2 and the mono-nuclear aromatic hydrocarbon is present in an amount of from about 20 to about 70 percent by Weight of total hydrocarbon.

4. A process according to claim 1 in which said dehydrocyclization catalyst is an alkali metal promoted Group VI metal oxide distended upon an inert carrier, and said contact is made at a temperature in the range of from about 900 to about 1200 F., a pressure in the range of from about one to about 50 p.s.i.g., and a space velocity of from about 0.3 to about 4 wt./hr./wt. based on total feed.

5. A process according to claim 4 in which said dehydrocyclization catalyst is sodium-promoted chromia on an alumina support, and said contact is made at a temperature in the range of from about 950 to about 1050 F., a pressure in the range of from about 5 to about 30 psig. and a space velocity of from about 0.5 to about 1.5 wt./ hr./wt. based on total feed.

6. A process according to claim 1 in which said nonaromatic hydrocarbon feed contains at least one member of the class consisting of n-hexane, n-hexenes, methylpentanes, methylpentenes, and methylcyclopentane and the mono-nuclear aromatic hydrocarbon is benzene.

7. A process according to claim 1 in which said nonaromatic hydrocarbon feed contains at least one member of the class consisting of n-heptane, n-heptenes, methylhexanes, and methylhexenes, and the mono-nuclear aromatic hydrocarbon is toluene.

8. A process according to claim 1 in which said nonaromatic hydrocarbon feed contains at least one member of the class consisting of n-octane, n-octenes, methylheptanes, methylheptenes, and dimethylhexanes in which the methyl groups are on different carbon atoms and the mono-nuclear aromatic hydrocarbon is o-xylene, m-xylene, p-xylene or mixtures thereof.

9. A process according to claim 1 in which said nonaromatic hydrocarbon feed contains at least one member of the class consisting of the ethyl hexanes and the ethyl hexenes and the mono-nuclear aromatic hydrocarbon is ethyl benzene.

10. A process according to claim 1 in which the mononuclear aromatic hydrocarbon is the same as the product aromatic hydrocarbon and is obtained by separating a portion of the product aromatic hydrocarbon from the reactor efliuent and recycling said portion to the feed to the reactor.

References Cited UNITED STATES PATENTS 3,299,156 1/1967 Ashley et a1. 260-6735 2,985,693 5/1961 Probst et a1. 260-6735 3,002,036 9/ 1961 Hieronymus 260-6735 FOREIGN PATENTS 534,236 3/1941 Great Britain.

DELBERT E. GANTZ, Primary Examiner J. M. NELSON, Assistant Examiner 

